This invention relates to a process for encapsulating chemically active substances, and particularly to encapsulating labile biological substances such as enzymes and hemoglobin, in a semipermeable membrane which will retain the encapsulated material, yet will freely allow smaller molecules to pass through the membrane to react with the encapsulated material and reaction products to pass out. It also relates to a method of controlling the size of the pores in the semipermeable membranes of the capsules.
Microencapsulation technology has grown rapidly in recent years and has found many applications. Attempts have been made, with varying degrees of success, to encapsulate biologically active substances such as hemoglobin, carbonic anhydrase, urease, asparginase, lactate dehydrogenase (LDH), glutamate oxaloacetate transaminase (GOT), chemically active materials such as ion exchange resins and activated charcoal, and various other enzymes and chemically active substances. Microcapsules of this nature have already proven to have great utility in, for example, artificial kidneys and fixed enzymes system, and show great promise in a number of other areas. As an example, U.S. Pat. No. 3,522,346 to Thomas M. S. Chang, discloses non-thrombogenic microcapsules which can be utilized, when fabricated about enzymes or detoxicants and placed in an extracorporeal shunt, to introduce oxygen, medicaments, enzymatic substances, and the like into the blood stream at controlled rates.
In Artificial Cells, by Thomas M. S. Chang, (Charles C. Thomas, Publisher; Springfield, Ill. 1972) a process for encapsulating biologically active materials in a semipermeable membrane is disclosed. The technique involves dissolving the material to be encapsulated and a monomer in water, and forming an emulsion with the water as the discontinuous phase. When a second monomer capable of polymerization with the first is dissolved in the continuous phase of the emulsion, a polymerization occurs at the interface of the two phases, and a membrane is formed around the typically colloidal sized droplets of solution.
This procedure has drawbacks and limitations which curtail its potentially wide applicability and commercial success. Since many biologically active substances, e.g. enzymes, are extremely labile, the relatively harsh reaction conditions necessary for polymerization often result in a very low yield of operable encapsulated material. Encapsulation of even relatively hardy enzymes, such as urease, is characterized by yields at best between 35 and 40 percent. At one or more steps in the process, much of the encapsulated material is denatured.
U.S. Pat. No. 3,522,346 (T. M. S. Chang) relates to microcapsules of controlled permeability. The process therein disclosed involves encapsulating aqueous compositions in membranes of controlled size, thickness, and permeability. Droplets of an aqueous composition are dispersed in an organic liquid medium immiscible therewith and a component soluble in the organic liquid medium and capable of reacting with a component of the hydrophilic composition is added to the dispersion thereby produced to form a macromolecular membrane by interfacial coacervation, polymerization, or condensation at the surface of the droplets. Typically, a polymer, condensation product, or a component thereof is added to the organic liquid and the membrane is formed about the droplets by interaction of the polymer, condensation product or component with a component of the dispersed droplets, which may be a precipitant, a condensation or polymerization catalyst, or a compound of the eventual condensation product. The effective pore size of the encapsulating membrane is a function not only of the membrane composition but also of the thickness of the membrane and is selected in accordance with the intended use of the capsules. By suitable choice of operating conditions, pore size may be varied, but the reaction condition necessary to ensure sufficiently strong yet selectively permeable membranes are in any case difficult to control and in some cases unknown. Also, the procedure produces capsules generally having membranes with erratic uniformity and porosity, many of which are useless for their intended purpose.
It is therefore an object of this invention to provide a process wherein a large variety of enzymes, other labile biological material, and many chemically active substances can be encapsulated in their active state, and to provide a higher yield of operative substance when so encapsulated.
A further object of the invention is to provide a method for forming very strong semipermeable membranes around any of a large number of labile biological materials.
Still a further object of the invention is to provide a method of manufacturing enzyme containing microcapsules wherein the microcapsule membrane has a more uniform degree of semipermeability, that is, to provide capsules with uniform walls without porous defects and with a pore size range such that the encapsulated material cannot escape from the interior of the capsule yet can readily react with substances in the environment in which the capsules are placed.
Another object of the invention is to provide a process for encapsulating a large variety of chemically active substances in a high quality semipermeable membrane by utilizing a variety of different polymer systems and different interfacial polymerization techniques. Yet another object of the invention is to provide a general process having some variable parameters, values for which may be selected to suit a particular substance to be encapsulated, to vary the nature of the capsule membrane, and to maximize the yield of operative encapsulated material.
Still another object of the invention is to provide a method of encapsulating a high concentration of hemoglobin in a capsule permeable to oxygen, carbon dioxide, and other small molecules.
Yet another object of the invention is to provide a method of controlling capsule membrane pore size to an improved degree, the method having wide applicability to a large variety of encapsulation techniques.
Still another object of the invention is to provide strong semipermeable microcapsules with membranes having pore sizes falling within any one of a number of size ranges, hence being selectively permeable to molecules having dimensions smaller than the upper limit of the range chosen.